Membranous nephropathy (MN) is one of the most common causes of nephrotic syndrome in adults. The J-RBR/J-KDR registry developed by the Japanese Society of Nephrology provides nationwide cohort data for epidemiological studies of MN. MN was present in 36.8% of 1,203 primary nephrotic syndrome patients in Japan. In addition, 633 (77.9%) out of 813 MN patients were referred to as "idiopathic," whereas 22.1% were classified as "secondary" and involved conditions such as systemic lupus erythematosus, drug exposure, infections, cancer, and various collagen diseases. The mean age of the MN patients was 62.2 (2-88) years old, their mean eGFR was 76.7 (7.6-154.6) ml/min/1.73 m(2), and 63.3% had hypertension at the time of renal biopsy. On the basis of these findings, half of Japanese idiopathic MN patients have risk factors (age >60, male, or lower eGFR) for end-stage renal failure, and 10% belong to the high-risk group (daily proteinuria of over 8.0 g). Further studies with high-grade evidence should resolve the natural history and therapeutic problems of idiopathic MN in elderly Japanese.